Neutrino mass from Cosmology

TL;DR

This paper surveys how relic and massive neutrinos shape cosmology, detailing their production, decoupling, and the resulting cosmic neutrino background. It explains how neutrino masses imprint distinctive signatures on the CMB and the matter power spectrum through free-streaming, affecting the growth of structure and the expansion history; these effects enable current and future bounds on the total neutrino mass, $\sum m_i$, and on the radiation content, $N_{ m eff}$. The authors synthesize constraints from CMB, BAO, H0, galaxy clustering, weak lensing, and Lyman-$\alpha$ data, highlighting that $M_\nu$ is constrained to the sub-eV level with planck-era data and next-generation surveys potentially detecting normal-hierarchy masses around $0.05$–$0.1$ eV. They emphasize the complementarity between cosmology and laboratory experiments (beta decay and neutrinoless double beta decay) and discuss future prospects, including CMB lensing and 21-cm cosmology, for probing the neutrino sector and possible sterile species.

Abstract

Neutrinos can play an important role in the evolution of the Universe, modifying some of the cosmological observables. In this contribution we summarize the main aspects of cosmological relic neutrinos and we describe how the precision of present cosmological data can be used to learn about neutrino properties, in particular their mass, providing complementary information to beta decay and neutrinoless double-beta decay experiments. We show how the analysis of current cosmological observations, such as the anisotropies of the cosmic microwave background or the distribution of large-scale structure, provides an upper bound on the sum of neutrino masses of order 1 eV or less, with very good perspectives from future cosmological measurements which are expected to be sensitive to neutrino masses well into the sub-eV range.

Figures (8)

• Figure 1: Photon and neutrino temperatures during the process of $e^{\pm}$ annihilations: evolution of their ratio (left) and their decrease with the expansion of the Universe (right).
• Figure 2: Evolution of the background energy densities in terms of the fractions $\Omega_i$, from the time when $T_{\nu}=1$ MeV until now, for each component of a flat Universe with $h=0.7$ and current density fractions $\Omega_{\Lambda}=0.70$, $\Omega_{\rm b}=0.05$ and $\Omega_{\rm cdm}=1-\Omega_{\Lambda}-\Omega_{\rm b} -\Omega_{\nu}$. The three neutrino masses are $m_1=0$, $m_2 = 0.009$ eV and $m_3 = 0.05$ eV.
• Figure 3: BBN contours at 95% C.L. in the $\eta_\nu-\eta_{\nu_e}^{\rm in}$ plane for several values of $\sin^2\theta_{13}$: 0 (solid line), 0.04 and normal mass hierarchy (NH) (almost vertical solid line), 0.04 and inverted mass hierarchy (IH) (dotted line) 20-Mangano:2011ip.
• Figure 4: Isocountours of the final value of $\Delta N_{\rm eff}$ in the $\sin^22\theta_s-\delta m_s^2$ plane for vanishing lepton asymmetry and $\delta m_s^2>0$ (left panel) and $\delta m_s^2<0$ (right pannel). The star denotes the best-fit mixing parameters as in the 3+1 global fit in 20-Giunti:2011cp: $(\delta m_s^2,\sin^22\theta_s)= (0.9~{\rm eV}^2, 0.089)$. Adapted from 20-Hannestad:2012ky. Courtesy of S. Hannestad, I. Tamborra, and T. Tram.
• Figure 5: Allowed values of the total neutrino mass as a function of the lightest state within the $3\sigma$ regions of the mixing parameters in eq. (\ref{['20-oscpardef']}). Blue dotted (red solid) lines correspond to normal (inverted) hierarchy for neutrino masses, where $m_0=m_1$ ($m_0=m_3$).